If there's one thing that Iago seems to have a nose for, it's another character's weak spot. With Cassio, the weakness is alcohol. In Act II, scene iii, Cassio is invited by Iago to drink heartily with his fellows:
. . .Come, lieutenant, I have a stoup of wine and here without are a brace of Cyprus gallants, that would fain have a measure to the health of black Othello.
I have very poor and unhappy brains for drinking. . .
. . .but one cup.
I ha' drunk but one cup tonight, and . . .behold what innovation it makes here: I am unfortunate in the infirmity, and dare not task my weakness with any more.
But, of course Iago convinces him to drink, which leads to a brawl, which leads to Othello stripping Cassio of his position.